Thermal treatment of thermoplastic filaments for the preparation of surgical sutures

ABSTRACT

Thermoplastic monofilament sutures having improved flexibility and handling characteristics are obtained by a melt spinning process which includes the step of drawing the filaments in a heated zone maintained at a temperature above the melting temperature of the filament. The resulting monofilament sutures have a higher elongation and lower modulus than comparable monofilament sutures obtained without the heated drawing step, and are characterized by a crystalline structure which is more highly ordered in the core of the monofilament suture than in a surrounding annular area.

This application is a division, of application Ser. No. 08/060,881,filed May 12, 1993, which is hereby incorporated by reference, now U.S.Pat. No. 5,294,395, which is a continuation of application Ser. No.690,140 filed Apr. 1, 1992, now abandoned, which is acontinuation-in-part of Ser. No. 402,092 entitled "Thermal Treatment ofThermoplastic Filaments", filed Sep. 1, 1989, now abandoned.

FIELD OF INVENTION

The present invention relates to the production of thermoplasticfilaments by extrusion and drawing for the preparation of surgicalmonofilament sutures, and more particularly, to a process for improvingthe physical properties of such thermoplastic monofilament sutures bythermal treatment. This invention also relates to surgical monofilamentsutures produced by the disclosed process.

BACKGROUND OF THE INVENTION

Monofilaments are prepared from many different thermoplastic polymersfor a variety of industrial and professional applications. The physicalproperties of such monofilaments, such as tensile strength, elongationand modulus, depend on the particular polymer composition and on themethod of manufacture. It is known, for example, that melt spun nylonand polyethylene terephthalate polymers produce monofilaments of highertenacity than, for example, the polyolefins, and that the tenacity ofsuch monofilaments is increased while elongation is decreased by drawingthe filaments under conditions to increase molecular orientation.

The use of thermoplastic monofilaments as surgical sutures is wellestablished. Monofilament sutures formed of isotactic polypropylene aredescribed in U.S. Pat. No. 3,630,205, and monofilament sutures ofpoly-dioxanone are described in U.S. Pat. No. 4,052,988.

Copolymers of p-dioxanone and glycolide useful in preparing monofilamentsutures are described in U.S. Pat. No. 4,635,497, while suturescomprising poly [tetramethylene terephthalate-co-(alkenyl or alkyl)succinate] are described in U.S. Pat. No. 4,388,426. Other polymerssuggested for use as synthetic absorbable sutures are disclosed in theliterature and are well-known to those skilled in the art.

It is generally desirable for surgical sutures to posses high tenacity(greater than 50,000 psi), low to moderate elongation (from 20-60%), andlow modulus (less than 500,000 psi). Low modulus values signify a highdegree of filament flexibility and limpness as opposed to a stiff, wirymaterial, a particularly desirable and even essential feature of asurgical suture. U.S. Pat. No. 3,630,205 describes a process whereby theflexibility of polypropylene sutures may be improved with littlesacrifice in tensile strength by stretching the monofilament about6.6×under controlled conditions and then relaxing to 76-91% of thestretched length.

U.S. Pat. No. 4,246,904 describes a surgical suture prepared from asegmented polyether-ester block copolymer which is reported to haveexcellent strength and flexibility compared to prior art monofilamentsutures. The sutures of poly(p-dioxane) described in U.S. Pat. No.4,052,988 are also characterized as possessing good tensile and knotstrength and a high level of flexibility and softness.

The desirability of suture flexibility and softness as indicated by lowmodulus values is well recognized by the surgical profession and is aphysical property constantly sought after in connection with thedevelopment of new surgical suture products. Some suture materials, suchpolylactide, polyglycolide, and copolymers of lactide and glycolide aregenerally considered to be too stiff and wiry to be used asmonofilaments in all but the smallest suture sizes, and have foundcommercial acceptance in the larger sizes only as braided sutures, whichare more flexible by virtue of their physical construction.

It is accordingly an object of the present invention to provide aprocess whereby monofilament sutures having improved softness andflexibility may be prepared. It is a further object of this invention toprovide a process for reducing the modulus of existing monofilamentsuture materials with little or no loss of tensile strength. It is yet afurther object of this invention to provide improved surgicalmonofilament sutures of poly(p-dioxanone) and other suitable polymericmaterials characterized by reduced values of Young's modulus. A furtherobject of this invention is to provide a method whereby the surfacecharacteristics of thermoplastic suture monofilaments may be modified toenhance the properties of such suture monofilaments, particularlysurface smoothness and integrity. These and other objects will beapparent from the ensuing description and claims.

SUMMARY OF THE INVENTION

In the basic embodiment of the present invention, drawn and orientedthermoplastic suture monofilaments are subjected to a heat treatment toreduce modulus and otherwise improve physical properties by passing thesuture filament through a radiant heater maintained above the meltingtemperature of the suture monofilament. Operating conditions arecontrolled so that the suture monofilament is subjected to a sufficienttime/temperature exposure to modify the near-surface crystallinestructure of the suture monofilament.

The suture monofilament is maintained under tension and preferably drawnslightly during the heat treatment. Draw ratios of 10-20 percent orhigher are possible with most materials. Treatment temperature may be 5°to 100° C. or more above the melting temperature of the suturemonofilament, with exposure time adjusted to obtain the desired effecton crystalline structure without penetrating too deeply within thesuture monofilament. Following the heat treatment, the monofilament isrelaxed and annealed to further increase crystallinity and decrease thedegree of amorphous orientation, and then sterilized to render themonofilament suitable for use as a surgical suture.

In a further embodiment of the present invention, melt extruded, liquidquenched suture monofilaments are drawn through a radiant heatermaintained at a temperature above the melting temperature of the suturemonofilaments. Draw speed, draw ratio, heater temperature and dwell timeare regulated to obtain the maximum stable draw ratio for the particularsuture monofilament material. This draw ratio will generally be 3×to6×and 10 to 30 percent more than the maximum stable draw ratioobtainable in the absence of the radient heater. The use of the radiantheater to increase the overall stretch imparted to the suture filamentduring the initial drawing and orientation step further improves theultimate physical properties obtainable by the method of this invention.The suture filaments thus produced are further processed according tothe basic embodiment of the invention.

Monofilaments, drawn and heat treated in accordance with the presentinvention, are further processed through relaxation and annealing, aswell as sterilization, following conventional procedures. The resultingmonofilament sutures are found to have significantly greater elongation,significantly lower modulus and substantially equivalent tensilestrength as compared to monofilament sutures processed without the heattreating step of the present invention.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic representation of a filament extrusion process forthe preparation of a surgical suture according to the present invention.

FIG. 2 is a schematic representation of a filament redraw process forthe preparation of a surgical suture according to the present invention.

FIG. 3 is a cross-section photomicrograph of a size 4/0 monofilamentsuture of poly(p-dioxanone) prepared according to the method of thepresent invention.

FIG. 4 is a cross-section photomicrograph of a size 4/0 monofilamentsuture of poly(p-dioxanone) prepared according to the prior art.

FIG. 5 is a cross-section photomicrograph of a size 6/0 monofilamentsuture of poly(p-dioxanone) prepared according to the method of thepresent invention.

FIG. 6 is a cross-section photomicrograph of a size 6/0 monofilamentsuture of poly(p-dioxanone) prepared according to the prior art.

DETAILED DESCRIPTION OF INVENTION

In one embodiment of the present invention, there is provided a processfor drawing and relaxing prepared suture filaments in order to modifyand improve the properties of such monofilament sutures. The suturefilaments subjected to such process have already been prepared byconventional methods of melt extruding, quenching, drawing, and relaxingcontinuous thermoplastic monofilaments. This embodiment of the inventionis referred to herein as the "redraw process".

In a second embodiment of the present invention, there is provided animproved process for the initial preparation of the suture filamentswhich are to be further processed by the redraw process. This embodimentof the invention, hereinafter referred to as the "extrusion process" incombination with the redraw process, constitutes a preferred embodimentof total invention and results in monofilament sutures having optimumproperties.

The extrusion process of the present invention is first described asfollows with reference to FIG. 1. The selected thermoplastic polymer ismelt extruded using conventional melt spinning apparatus indicatedgenerally as 10. The polymer passes from the spinnette 11 having one ormore jet orifices sized to provide the correct final diameter of thedesired monofilament into aqueous quench bath 12 where the moltenpolymer stream hardens into filament F. The filament passes aroundsubmerged roll 13 and exits the quench bath over roll 14, thereafterpassing into air cabinet 15 where the filament is dried and conditionedbefore entering the drawing portion of the process. The roll speed incabinet 15 may be adjusted to take up the filament at a rate faster thenthe filament is being extruded from the orifice in which case the rollsimpart a "jet-stretch" to the filament. The amount of jet stretch,together with orifice size, is adjusted to produce a monofilament of thedesired diameter.

The filaments pass from air cabinet 15 to draw rolls 16 which aregenerally maintained at the same peripheral speed as the rolls in thecabinet so the filaments are maintained under tension but notsignificantly drawn at this point. The filaments leaving draw rolls 16pass through tube furnace 17 onto draw rolls 18 which operate at ahigher peripheral speed than draw rolls 16 in order to impart a highdegree of stretch to the filaments, i.e., 3×-6×(200-500%). The tubefurnace, which is optional at this point in the present invention, isoperated at a temperature of from 5° to 100° above the meltingtemperature of the filaments, and most generally, from 20° to 75° abovethe melting point. Because the furnace is relatively short and the timeof filament exposure is short, only the surface of the filament isaffected by the heat. The use of the tube furnace at this point in theprocess allows a higher degree of stretch to be imparted to thefilament, but for some polymer systems, the furnace may be omitted andthe filament given a cold, warm air or hot water stretch. In any case,the drawing at this stage imparts molecular orientation to the filamentswhich are essentially amorphous or unoriented spherolitic structuresgoing onto rolls 16.

After drawing, the filaments pass through hot air orientation oven 19,maintained at a temperature between the Tg and the melting point of thepolymer, where they are given a small additional stretch of e.g. 10-25%by draw rolls 20. This completes the initial filament preparation stageof the process and the filaments are spooled and stored for furtherprocessing in the redraw process of the present invention.

The redraw process is next described with reference to FIG. 2 asfollows. Filaments F prepared and spooled in the extrusion process orobtained from other suitable source are fed to draw roll 21, throughtube furnace 22, and onto draw rolls 23. The filament is drawn usuallyless than 100% and most generally from 10 to 25% between rolls 21 and23. The tube furnace, which is an essential component of the redrawprocess, is operated at a temperature at least 5° C. above the meltingpoint of the filaments, and preferably at least 15° or more above themelting point. In general, the maximum temperature which is consistentwith good operation is preferred and the optimum temperature for anygiven process will be dependent on filament size and composition,operating speed, and draw ratios. Such optimum temperatures are readilydetermined by experimental procedures for any given system.

The drawn and heat treated filament proceeds from rolls 23 through hotair conditioning oven 24 and on to draw rolls 25 which operate at aperipheral speed slightly above that of rolls 23, e.g. 2-10%, in orderto maintain the filaments under tension. The filaments next proceed fromrolls 25 through hot air relaxing oven 26 and onto rolls 27. Rolls 27operate at a peripheral speed which is 10 to 30% slower than that ofrolls 25 in order to permit the filaments to shrink a controlled amountwhile passing through oven 26. The temperatures in oven 24 and oven 26are maintained above the Tg but below the melting temperature of thefilament, usually about 5° to 20° C. below the melting point.

Filaments leaving rolls 27 are collected and optionally subjected tofurther processing if desired, to enhance physical properties of thefilaments such as annealing to increase crystallinity. After theseadditional optional steps, the filaments are sterilized so the filamentsare suitable for use as a surgical suture. Sterilization can be carriedout, for example, by subjecting the filaments to cobalt 60 radiation orethylene oxide vapor. Other sterilization means can be used if desired.

The tube furnace, which is an essential component of the redraw process,and an optionally preferred component of the extrusion process, ispreferably a high temperature furnace which heats by radiant energy aswell as convection. A suitable furnace for single or small groups ofmonofilaments is tube furnace Model No. 55035 available from theLindberg Co., 304 Mart Street, Watertown, Wis. This furnace is designedto reach temperatures of up to 900° C., and is heated by electricalelements backed by a refractory material. The furnace has a 1-inchdiameter tubular opening through which the filaments pass. It isimportant when using this furnace that the filaments be centered in theopening to avoid uneven heating. For larger scale multifilament sutureproduction, a furnace with infrared heaters and parallel heating plategeometry would be preferred to a tubular furnace.

The method of the present invention is illustrated by the followingexample describing the preparation of a monofilament suture of poly(p-dioxanone) . Poly (p-dioxanone) sutures are known from U.S. Pat. No.4,052,988, incorporated herein by reference. The preparation ofp-dioxanone monomer and polymerization of that monomer to obtainpolymers having intrinsic viscosities in excess of 0.80 are described inthis reference. Further, the extrusion of the polymer according toExample VI of this reference resulted in monofilaments having a diameterof 11 mils (corresponding to a 3/0 suture) a tensile strength of 55,600psi, a dry knot strength of 48,800 psi, and a Young's modulus of 167,000psi. Example VII describes a 9 mil monofilament having a tensilestrength of 70,600 psi, dry knot strength of 50,300 psi, and anelongation of 46.3 percent.

A commercial monofilament suture comprising a polymer of p-dioxanone andmanufactured and sold by Ethicon, Inc., Somerville, N.J., under thetrademark "PDSβ Suture", has been determined to have average propertiesas shown in Table I below.

Poly(p-dioxanone) sutures size 2/0 and 5/0 were produced from the samepolymer as aforedescribed commercial sutures by the method of thepresent invention. The physical properties of these sutures are alsopresented in Table I below. As illustrated by these data, the suturesproduced according to the present invention have similar tensile andknot strength, greater elongation, and significantly lower modulus thanthe commercial sutures.

                  TABLE I                                                         ______________________________________                                        Suture Properties                                                                     This Invention                                                                            Commercial Product                                        Suture Size                                                                             5/0      2/0      5/0      2/0                                      ______________________________________                                        Diameter (mil)                                                                          6.8      13.5     6.5      13.7                                     Tensile (lb)                                                                            3.4      12.6     2.9      11.1                                     Intr. ten. (psi)                                                                        92,770   88,775   87,550   75,010                                   Elong. (%)                                                                              58.0     60.0     38.1     34.1                                     Knot (lb) 2.3      7.3      2.1      7.0                                      Intr. knot (psi)                                                                        63,130   51,280   62,000   47,140                                   Modulus (psi)                                                                           approx. 200,000                                                                             approx. 330,000                                       ______________________________________                                    

The reduced modulus and increased elongation of the sutures of thepresent invention impart improved handling characteristics and reducedtissue drag which are readily perceived and preferred by surgeons usingthese materials. In particular, the reduced modulus results in greaterflexibility and less springiness than that characteristic of thecommercial product. The poly(p-dioxanone) sutures of the presentinvention were found to have essentially the same in vivo breakingstrength retention and absorption characteristics as the commercialproduct.

The sutures of the present invention reported in Table I were preparedby the process as generally described below. This process is describedspecifically for the manufacture of poly(p-dioxanone) sutures whereinthe starting polymer has an average molecular weight of from about65,000 to 100,000, an inherent viscosity of from about 1.60 to 2.20, anda melting temperature of from about 95° to 105° C. Process conditionsmay vary for other polymer compositions including otherpoly(p-dioxanone) polymers and the following example is accordinglypresented for purposes of illustration only as being representative forone specific polymer system and is not otherwise limiting of the presentinvention. Many variations in the process equipment and operatingparameters which nevertheless incorporate the essential features of thepresent invention will be apparent to those skilled in the art and areintended to be included within the scope of the present invention.

EXAMPLE (size 2/0)

Poly(p-dioxanone) polymer having an average MW of about 78,000, aninherent viscosity of 1.65-1.80 (melting point approx. 95° C.) was meltextruded through a 60 mil orifice and quenched in a water bath at 24° C.The extruded filament was conditioned to about 80° C. in a warm aircabinet and fed to a draw station where it was drawn 5×between two setof godets while passing through a tube furnace at 125° C. The firstgodet speed was 13 FPM, the second godet speed was 65 FPM, and thefurnace was 12 inches long. Filament dwell time in the furnace wasestimated to be about 1 second. The drawn filament was drawn anadditional 20% while passing through a hot air oven maintained at about80° C. The filament was collected and conditioned under vacuum at roomtemperature for 16 hours. The monofilament was next drawn 15% whilepassing through a 12 inch tube furnace maintained at about 115°-120° C.Dwell time in the furnace was estimated to be approximately 1.5 seconds.The drawn filament was next passed through a hot air oven maintained atabout 90° C. with a 4% stretch to maintain tension, followed by a secondoven at 90° C. wherein the filament was allowed to shrink 15%. Theresulting filament was wound on a rack for annealing at a temperature offrom about 70° to 90° C., and, in the present example, at 85° C. for 6hours while maintained under tension to prevent further shrinkage. Theresulting monofilament had a diameter of 14 mils corresponding to a size2/0 surgical suture.

The above procedure was repeated to prepare sutures ranging in size from7/0 (0.4 mils) to 2 (23 mils). Individual suture sizes were obtained byadjusting extrusion rates and/or orifice size to produce an extrudedmonofilament which, after passing through the remainder of the processwhich resulted in a 6×overall stretch, would produce the desired suturediameter. While minor adjustments in process speeds and temperatureswere made to accommodate the different filament sizes and to optimizeprocess conditions for each size, the basic process was the same as thatdescribed above for the size 2/0 suture.

Representative physical properties for sutures prepared in accordancewith the present invention as described above are presented in Table IItogether with comparable data for commercial sutures of the samepolymeric compositions. As noted above, the commercial sutures wereprepared by a conventional process which did not include heat treatmentat a temperature above the melting point of the polymer.

                                      TABLE II                                    __________________________________________________________________________    Physical Properties of Sutures of Poly(p-dioxanone)                           This Invention        Commercial Product                                          Straight                                                                           Knot         Straight                                                                           Knot                                               Suture                                                                            Tensile                                                                            Strength                                                                           Elong.                                                                            Strain                                                                            Tensile                                                                            Strength                                                                           Elong                                                                             Strain                                    Size                                                                              (lbs)                                                                              (lbs)                                                                              (%) Load                                                                              (lbs)                                                                              (lbs)                                                                              (%) Load                                      __________________________________________________________________________    2   28.1 16.4 59.4                                                                              2.7 29.9 16.1 37.1                                                                              3.9                                       1   22.8 12.9 54.7                                                                              2.2 24.5 13.2 27.1                                                                              3.1                                       0   17.8 10.3 58.3                                                                              1.6 17.6 10.5 36.9                                                                              2.4                                       2/0 11.0 7.2  52.0                                                                              1.0 10.9 6.8  32.3                                                                              1.8                                       3/0 7.7  5.5  51.6                                                                              0.8 8.1  5.2  34.5                                                                              1.2                                       4/0 4.6  3.3  53.7                                                                              0.4 4.8  3.2  32.1                                                                              0.7                                       5/0 2.9  2.2  44.6                                                                              0.3 3.0  2.1  33.3                                                                              0.4                                       6/0 1.3  1.0  50.6                                                                              0.1 1.3  0.9  29.1                                                                              0.2                                       7/0 0.8  0.7  52.6                                                                              0.1 0.6  0.6  26.1                                                                              0.1                                       __________________________________________________________________________

As illustrated by the data in Table II, which are generallyrepresentative of these products, sutures prepared according to thepresent invention are substantially equivalent to the commercial suturesin straight tensile and knot strength, but have significantly higherelongation. In addition, the sutures of the present invention possess ahigher degree of elasticity as indicated by the Strain Load which is ameasure of the force required to reach the 3% strain level in astress-strain test. The lower the strain load value, the more stretchythe suture is perceived to be.

Sutures of poly(p-dioxanone) were also evaluated for tensile strength,modulus, and Gurley Stiffness with the representative results presentedin Table III below.

                                      TABLE III                                   __________________________________________________________________________    Physical Properties of Sutures of Poly(p-dioxanone)                           Nominal                                                                            This Invention         Commercial Product                                Suture                                                                             Diameter                                                                           Gurley                                                                             Ten.                                                                             Mod.      Diameter                                                                           Gurley                                                                             Ten.                                                                             Mod.                                 Size mils Stiffness                                                                          (psi × 10.sup.-3)                                                              Ten./Mod.                                                                           mils Stiffness                                                                          (psi × 10.sup.-3)                                                              Ten./Mod.                        __________________________________________________________________________    2    23.3 240  63.7                                                                             210 0.30  23.8 364  72.5                                                                             336 0.22                             1    20.8 149  -- --  --    19.6 169  75.7                                                                             318 0.24                             0    18.4 85   69.1                                                                             200 0.35  17.0 118  71.8                                                                             323 0.22                             2/0  14.1 30   70.2                                                                             198 0.35  13.6 54   73.4                                                                             337 0.21                             3/0  12.0 16   74.2                                                                             204 0.36  11.6 23   75.3                                                                             333 0.23                             4/0  8.4  5.6  -- --  --    8.7  7.0  82.3                                                                             354 0.23                             5/0  6.6  1.8  82.6                                                                             199 0.43  6.9  2.6  77.2                                                                             334 0.23                             6/0  4.4  0.2  -- --  --    4.4  0.2  89.6                                                                             322 0.28                             7/0  0.4  0.04 -- --  --    0.4  0.04 -- 351 --                               __________________________________________________________________________

As shown by the above data, the sutures prepared according to thepresent invention have significantly lower modulus than the commercialproduct, approximately 200,000 psi vs. 330,000 psi. This lower modulusis also reflected in the Gurley Stiffness values which are significantlylower for the sutures of the present invention compared to thecommercial product. In this particular study, the tensile strength ofthe test sutures was slightly lower than that of the commercial product.Further process refinement and optimization of operating conditionshowever, would be expected to produce further improvement of thephysical properties of sutures prepared according to the presentinvention, including increased tensile strength. Particularly preferredsutures of poly(p-dioxanone) prepared according to the present inventionare specifically characterized by the following physical properties:

Elongation--greater than 40%

Young's Modulus--less than 250,000 psi

Tenacity/Modulus ratio--greater than 0.30

Test methods used for determining the physical properties ofmonofilament suture materials were as follows:

Tensile strength, knot strength and percent elongation at break weredetermined by A.S.T.M. method D-2256-66T at a constant rate of extensionusing an INSTRON table Model 4200 universal testing instrumentmanufactured by the Instron Corporation of Canton, Mass. With theinstrument sample clamps set 5 inches apart, 5-inch lengths of suturewere elongated at a rate of 12 inches per minute until fracture. Thesuture knot strength was determined under the same test conditions.

The INSTRON instrument was set for the correct suture diameter, andYoung's Modulus was calculated in psi from the initial stress-straindata generated during the straight tensile strength test. Young'sModulus is the ratio of applied stress to strain in the elastic regionof the suture and measures the elastic component of a suture'sresistance to stress. This value is related to the flexibility of asuture.

Gurley stiffness was measured using a motor-operated Gurley StiffnessTester (Model 4171-D) manufactured by Teledyne-Gurley of Troy, N.Y. Inthe test, 10 parallel monofilaments are mounted on a fixture so that thefilaments extend precisely 3/4 inch beyond the edge of the fixture. Theinstrument is adjusted to provide 1/2 inch clearance between the edge ofthe fixture and the pendulum so that the pendulum acts against 1/4 inchof the monofilaments. The instrument is operated and filament stiffnesscomputed according to the manufacturer's instructions.

Good suture flexibility as indicated by low Gurley Stiffness values isimportant to the surgeon since soft, highly flexible sutures are easierto handle, use and tie. Flexible and slightly elastic sutures are alsodesirable since they conform to the wound and permit some latitude inthe tension applied to the suture by the surgeon.

In addition to their desirable physical properties, monofilament suturesproduced according to the present invention are characterized by aunique morphology which includes a central core having a highly orderedcrystalline structure and an annular portion between the central coreand outer surface which has a less ordered crystalline structure. Thisdifferentiation in crystalline structure is believed to be the result ofexposing the suture filament to high temperatures in excess of themelting point of the polymer for brief periods during the manufacturingprocess as described above. While applicant does not wish to be bound bytheory, it is postulated that the surface of the suture filament mayactually melt as the filament passes through the heater section, and thepenetration of heat into the body of the filament results in thedifference in crystalline structure noted above. The thickness of theannular portion which may be from about 0.1 to 0.5×the radius of thesuture filament is a measure the degree of heat penetration which is afunction of the time and temperature of exposure.

The morphological differences between the poly(p-dioxanone) monofilamentsutures produced according to the present invention and the commercialsutures of the same polymeric composition are readily apparent in FIG. 3and FIG. 4, which are 430×photomicrographs of cross-sections of size 4/0filaments illuminated with polarized light. The commercial productillustrated in FIG. 4 is seen to have a relatively uniform structurethroughout its cross-section. The monofilament suture of the presentinvention illustrated in FIG. 3 is seen to have a clearly definedcentral core surrounded by annular area A which extends inward from thesurface of the filament a distance equal to about 0.2×the radius of thesuture. The crystalline structure of the central core appears to behighly ordered and characterized by spherolitic monphology. The annulararea surrounding the core appears to show nonspherolitic morphology.This observed difference in crystalline structure is theoreticallyconsistent with the effect of the different processing conditions usedin the manufacture of the two products. The same effect is seen in FIG.5 and FIG. 6 which are respectively 430×photomicrographs of size 6/0sutures of the present invention and commercial product. In FIG. 5,annular area B extends inward from the surface of the suture a distanceequal to about 0.4×the radius of the suture.

The differences in crystalline structure between the poly(p-dioxanone)sutures of the present invention and the commercial product are readilydetermined by x-ray diffraction patterns and scattering analysis asillustrated by the representative values shown in Table IV.

                  TABLE IV                                                        ______________________________________                                        Crystalline Properties of Poly(p-dioxanone) Sutures                                  This Invention                                                                              Commercial Product                                       Suture Size                                                                            X.sub.c                                                                              L      l.sub.s                                                                           F.sub.h                                                                             X.sub.c                                                                            L   l.sub.s                                                                             F.sub.h                       ______________________________________                                        2/0      0.52   78     95  206   0.47 64  96    229                           4/0      0.55   72     97  208   0.52 61  97    245                           6/0      0.56   76     97  180   0.51 71  99    213                           ______________________________________                                    

where:

X_(c) is the relative crystallinity calculated as

    X.sub.C =ΣI.sub.c /ΣI.sub.t

where ΣI_(c) is the integrated area under the crystalline peaks, andΣI_(t) is the integrated area under the observed diffraction curve.

L is an estimate of the breadth in Å of individual crystallites usingthe Debye-Scherrer equation

    L=λ/βcos Θ

where Θ is half the diffraction angle, λ is the wave length of radiationin Å, and β is the peak width in Å at half-height.

l_(s) is a measure of the long range crystalline structure determined bysmall angle x-ray scatter as the average distance in Å from the centerof a lamella to the center of the next measured along the fiber axis.

F_(n) is a measure of the long range crystallite perfection in the chaindirection along the fiber axis as indicated by crystalline peak width inÅ at half-height using the Debye-Scherrer equation as defined above.

The method of the present invention has been illustrated with specificreference to monofilament sutures of poly(p-dioxanone) having uniquemorphology and improved physical properties, and such sutures are partof the present invention. The method of the present invention however,has wide application for the preparation of monofilament sutures usedwith other thermoplastic polymeric compositions. Such polymericcompositions which are contemplated for use in the method of the presentinvention include polyethylene, polypropylene, polyvinylidene fluoride,poly(glycolide, cotrimethyl carbonate), homopolymers and copolymers oflactide and glycolide and copolymers of p-dioxanone with lactide,glycolide and e-caprolactone.

Many variations in the details of the process of the present inventionwill be apparent to those skilled in the art and may be made withoutdeparting from the spirit and scope thereof. The process may, forexample, incorporate further drawing, relaxing and annealing stages inaddition to those disclosed herein or may omit or modify one or more ofthe nonessential processing steps described herein. In addition, theessential feature of the present invention which involves heat treatinga suture monofilament comprising a thermoplastic polymer at atemperature in excess of the melting point of that polymer, is subjectto variation according to the composition of the polymer, the size ofthe monofilament, and the time/temperature relationship of the heattreating step. For example, while the preceeding description includedtemperatures as high as 75° C. above the melting point of the polymer,and exposure times as brief as 0.1 seconds, it may be desirable forhigher production speeds or for other purposes to reduce exposure timesto 0.01 seconds or less while increasing the temperature up to severalhundred degrees above the melting point of the polymer as necessary toobtain the desired effect on the morphology of the monofilament sutureand its physical properties. It is accordingly understood that thisinvention is not limited to the specifically described embodimentsexcept as may be defined in the claims appended hereto.

I claim:
 1. A drawn and oriented monofilament suture comprising athermoplastic polymer selected from the group consisting ofpoly(p-dioxanone), polyethylene, polypropylene, polyvinylidene fluoride,copolymers of glycolide and trimethylene carbonate, homopolymers oflactide, homopolymer of glycolide, copolymers of glycolide and lactide,copolymers of lactide and p-dioxanone, copolymers of glycolide andp-dioxanone, and copolymers of ε-caprolactone and p-dioxanone; having aunified structure defined by an outer surface, a central core, andannular portion between said central core and said outer surface, saidcentral core having a spherulitic structure which is different from thatof said annual portion.
 2. The monofilament suture of claim 1 whereinsaid differentiation between said central core and said annular portionis readily visible by optical microscopy at 430×magnification underpolarized light.
 3. The monofilament suture of claim 1 wherein thecrystalline structure of the center core is characterized by largercrystals as compared to the annular portion.
 4. The monofilament sutureof claim 1, wherein the monofilament suture has a radius and the annularportion comprises in the range of from about 0.1 to 0.5×of the radius ofthe monofilament suture.
 5. The monofilament suture of claim 1 whereinsaid polymer is poly(p-dioxanone).
 6. The monofilament suture of claim 5wherein said polymer has an average molecular weight in the range offrom about 65,000 to 100,000.
 7. The monofilament suture of claim 6,wherein the crystalline structure of the central core has a spheruliticstructure that is visible by optical microscopy at 430×magnificationunder polarized light and the annular portion does not show aspherulitic structure under the optical microscopy at 430×magnificationunder polarized light.
 8. The monofilament suture of claim 7 wherein themonofilament suture has a radius and the annular portion comprises inthe range of from about 0.1 to 0.5×the radius of the monofilamentsuture.
 9. The monofilament suture of claim 5, having an elongationgreater than 45%, a Young's modulus of less than 250,000 psi, and aratio of Tenacity/Modulus greater than 0.30.
 10. A monofilament sutureof claim 1 having a diameter in the range of from about 0.3 to 24 mils.11. A monofilament suture of claim 1 having a diameter in the range offrom about 0.3 to 24 mils.
 12. A monofilament suture of claim 5 having adiameter in the range of from about 0.3 to 24 mils.